Monitoring the Relationship between the Humerus and Clavicle Angles as Indicators for Neuromuscular Disorders

To predict neuromuscular disorders, we are interested in monitoring upper limb movements continuously using wearable sensors. Shoulder muscle coordination is complex and for those with neuromuscular disorders, this coordination often fails. As monitoring the entire kinematics of the shoulder complex is nearly impossible to do continuously, we propose to simply track the humerus and clavicle angles using wearable and portable sensors as a way to continuously monitor and/or predict neuromuscular disorders. For healthy subjects, we found that the relationship between the elevation angles of the humerus and the clavicle are correlated with second order polynomials. Furthermore, this relationship is consistent for a given individual and is repeatable. Although the fits obtained differ slightly from subject to subject, the general shape of the fit is consistent across different individuals. Knowing the relationship between the elevation angles of the humerus and the clavicle for healthy shoulder movement, we will be able continuously to identify abnormal shoulder movements while patients participate in everyday activities. INTRODUCTION Abnormal movement jerkiness, velocity profile, tremor, and muscular coordination have all been reported to be observable symptoms of neurodegenerative disorders such as Parkinson’s, Huntington’s, or Multiple Sclerosis [1, 2]. These abnormal movements are also observed for patients recovering from strokes or spinal cord injuries. Although there are many ways to evaluate and monitor these neuromuscular disorders through tracking limb movements, we have chosen to do so by having patients use wearable and portable sensors to allow continuous observation. As a first step toward this goal, we propose to track shoulder movement coordination. Shoulder movement is crucial for manipulation, and the complex muscles and joints of the shoulder must operate in harmony. Studies have shown that neuromuscular disorders can distort this relationship, reduce the range of motion of the joints, or change the average velocity of movements [3]. Tracking the entire shoulder complex, which is a combination of sternoclavicular, acromioclavicualar, scapulothoracic and glenohumeral joints having at least 14 degrees of freedom, is not realistic particularly if the movement needs to be measured continuously during normal activities. Attempting to simplify the representation of shoulder movements, Zatsiorsky [4] has shown that shoulder movement can be efficiently represented with 7 degrees of freedom, but that is still a large number of degrees of freedom to monitor. Our goal is to identify a distinctive and observable shoulder movement relationship that is consistent for healthy subjects but that fails for patients with neuromuscular disorders. To this end, we propose to isolate the observation to the elevations of the humerus and clavicle and understand the relationship between them in healthy subjects. METHODS Our wearable system consists of accelerometers that can track the movements as well as the orientation of the limbs. Figure 1 shows a Figure 1. Our wearable and portable prototype system to track limb movements. subject wearing our first prototype that uses a DC-coupled capacitive tri-axial accelerometer (Kistler, Inc. model 8392B10 K-Beam). It has a range of 10g, weighs 42 grams and responds well to temperature variations. For our experiment in this paper, one accelerometer was placed immediately proximal to the elbow joint with Velcro. The gravitational vector is used to indicate the elevation of the humerus. To validate the accuracy of our prototype and the shoulder model relations presented in this paper, an OptoTrak Motion Analysis System (Northern Digital, Inc.), which has a RMS positional accuracy of 0.1mm, was used. A total of three 6-Marker Probe markers were placed on the subject: One marker was placed on the forearm next to the accelerometer and the other on the clavicle. The frame of reference marker was placed at the top of the sternum. We recorded shoulder movements from six subjects with no known neuromuscular disorders. With the accelerometer and the markers attached to their right arms, we asked subjects to lift their right arms five times at three different values of humerus abduction angles: 0 (straight out in front), 45 , and 90 (straight out to the side). To track the markers, we used the rotation matrix for each marker returned by the OptoTrak. A rotation matrix, R, is a 3 3 matrix that makes up a frame in terms of another frame such that: